The production of plastics containing isocyanurate groups and urethane groups by reacting polyesters containing at least two hydroxyl groups and phthalic acid residues with an excess of polyisocyanates in the presence of trimerization catalysts is known.
Thus, a mixture of various esters which contain phthalic acid residues are used for the production of foams containing isocyanurate groups in accordance with the teaching of the German Pat. No. 1,112,285 (cf. Example 13). However, the proportion of phthalic acid in the carboxylic acids, which are used for the production of the esters, is below 50%, by weight, in this case.
The use of polyesters which only contain small quantities of phthalic acid residues for the production of plastics containing isocyanurate groups is also disclosed in German Offenlegungsschrift No. 1,769,023. The production of a foam containing isocyanurate groups from these polyesters, having a phthalic acid residue content of below 50%, by weight, is again described in German Offenlegungsschrift No. 1,794,117.